This invention pertains to the electrical signal processing art and, more particularly, to a method and apparatus for the selection of one of several redundant signals.
Redundant channel signal processing is well known to the prior art. A particular example is the aviation art, wherein several sensors each sensing the same airplane parameter provide corresponding output signals. Processing circuitry receives the redundant signals, and includes a means to select that signal, or a combination of signals which is most likely to be representative of the actual airplane parameter.
In one prior art approach to redundant signal processing, the signal output by the processing network is that signal having a midvalue between the other input signals. Equalization of all signals, to eliminate null offsets among the various sensors, is accomplished through a first order lag circuit. Signal monitoring detects a fault on a channel and, upon such fault substitute a zero for the failed input signal.
This approach suffered numerous deficiencies. First, lag circuit equalization did not completely eliminate failure transients. Second, the output incurs a dead zone about zero after isolation of a first failure. This dead zone resulted from the fact that for substantial time periods the zero substituted signal is the midvalue signal, with this midvalue zero signal being outputted as representative of the parameter being monitored. Finally, fault monitoring in this system is less than optimum.
In a second approach, an output is produced which is the average of the inputs. A defect of this approach is that it depends heavily on the signal monitoring to protect against failure transients. Large control error transients can occur before faults are detected and isolated. Complex schemes are devised to take questionable signals out of the averaging process as soon as monitoring thresholds are exceeded, and then monitoring a time delay to determine if the signal should be locked out permanently or returned to the averaging process. Lag equalization is sometimes provided to allow using smaller monitoring thresholds, but the lag equalization in combination with the average selection has the deficiency of allowing slowover faults to cause large errors in the output before the fault can be detected.
In a third prior art approach, the output is selected as a master channel with monitoring causing the system to switch to an alternate channel if the master channel fails. This process is limited in that the monitoring cannot provide 100% protection against first faults. Further, in requiring that a master channel exist, selection and control can not be identical on all three channels.
In a final approach, the output is selected as the midvalue of inputs and integral equalization is used to completely eliminate static offsets between the signals. However, a defect in this system has been that the equalizing integrators can drift allowing the output signal to go beyond boundaries defined by the nonfailed input signals. This results in an output signal which is not truly representative of the parameter being sensed.